The invention in general relates to magnetic resonance applications such as nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI), and in particular to a cavity resonator for applying a radio-frequency (RF) magnetic field to a target of interest.
Nuclear magnetic resonance (NMR) spectrometers and magnetic resonance imaging (MRI) devices typically include a superconducting magnet for generating a static magnetic field B0, and a special-purpose radio-frequency (RF) coil for generating a time-varying magnetic field B1 perpendicular to the field B0. In NMR applications, the RF coil is typically provided as part of an NMR probe, and is used to analyze samples situated in test tubes or flow cells. For typical MRI applications, the RF coil is used to analyze tissue or part of a patient. The direction of the static magnetic field B0 is commonly denoted as the z-axis, while the plane perpendicular to the z-axis is commonly termed the x-y or xcex8-plane. In the following discussion, the term xe2x80x9clongitudinalxe2x80x9d is used to refer to the z-direction, while the term xe2x80x9ctransversexe2x80x9d is used to refer to the xcex8-direction.
The RF coil resonates at the Larmor frequency of the nuclei of interest. A commonly used RF coil geometry is the xe2x80x9cbirdcagexe2x80x9d geometry, described for example by Hayes in U.S. Pat. No. 4,692,705. Hayes describes a coil including a pair of generally transverse conductive endcap elements. The endcap elements are separated along the longitudinal direction. A plurality of longitudinal segments interconnect the endcap elements, and are spaced apart along the peripheries of the endcap elements. Each of the segments has a reactive element connected in series therewith.
Hayes teaches that a perfectly uniform magnetic field B1 can, in principle, be produced in an infinitely long cylinder with a surface current parallel to the cylinder axis and proportional to sin, where is the polar position along the circumference of the cylinder. For a short coil, however, the field is not uniform near the coil ends. Hayes teaches that it is theoretically possible to construct a short coil with the uniformity of an infinitely long coil, by intersecting the coil with an infinitely long conductive sheet perpendicular to the cylinder""s axis. Hayes concludes that such a coil design is not practical, however, and proposes his birdcage geometry. The current through the longitudinal segments of Hayes follows a step-wise quasi-sinusoidal pattern, as illustrated in FIG. 5B of Hayes.
Other U.S. Patents describing RF resonator designs include U.S. Pat. No. 5,445,153 (Sugie et al.), U.S. Pat. No. 4,680,548 (Edelstein et al.), U.S. Pat. No. 5,144,240 (Mehdizadeh et al.), U.S. Pat. No. 5,202,635 (Srinivasan et al.), U.S. Pat. No. 5,212,450 (Murphy-Boesch et al.), and U.S. Pat. No. 4,480,239 (Hyde et al.). Information on the theory of birdcage resonators can be found in articles by Tropp, xe2x80x9cThe Theory of Birdcage Resonators,xe2x80x9d J. Mag. Resonance, 82:51-62 (1989), and Pascone et al., xe2x80x9cGeneralized Electrical Analysis of Low-Pass and High-Pass Birdcage Resonators,xe2x80x9d Mag. Resonance Imaging, 9:345-408 (1991).
The present invention provides a radio-frequency cavity resonator for nuclear magnetic resonance applications such as NMR and MRI. The resonator includes an outer conductive cylindrical shell, and an inner conductive re-entrant cylindrical shell laterally enclosed by the outer shell. The inner shell extends along the ends of the outer shell but not along a middle region of the outer shell. A target region for placing the magnetic resonance target of interest is defined in the middle region of the outer cylindrical shell. The space between the inner and outer shells may be filled by a dielectric.
The resonator is driven capacitively or inductively, for example by placing excitation loops in the space between the outer and inner shells. The resulting azimuthal current distribution is truly (continuously) sinusoidal, and the transverse magnetic field B, is spatially homogeneous within the target region. The design is relatively simple to construct and allows high Q-factors, wide tuning ranges for given cavity sizes, and relatively homogeneous transverse magnetic fields. The present invention further provides multiply-resonant cavity resonators comprising one or more intermediate conductive shells situated between the inner and outer shells.